Went from about 300 miles per tank to getting 150 after changing sensor.

Sensor is correct part #
I noticed an issue before- now gone- where after my car was warm, RPM's would creep high- in the the 1000 to 1200, sometimes 1500 range when I would shift the auto tranny from D to N, for instance.
Now that's gone and the RPM's seem settled at about 800 while in gear and stay that way when shifting around from P to N to D etc...
Exaust seems to smell awful now. Before new O2, seemed better. No clouds of smoke or anything.
AMM has been working fine.
Plugs have maybe 8-9 k on them now
Cleaned IAC recently.
Cleaned TB w/new gasket recently.
Gas gage seems to work fine. (Temp gauge does stick occaisionally, requiring a thump of the dash board)
Tank does not seem to be leaking anywhere.
Both fuel pumps seem to work.

Other than someone siphoning gas from my vehicle (possible), I don't know why I would see this major jump. And at $4/gal it's pretty evident this can not continue.

Where to start? Thanks guys...
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89 245 'loaded' with a Great Pyrenees

I had this happen to me on my 1989. First new sensor from FCP was bad right out of the box. Eventually, your spark plugs will start to foul out, and your car will start idling rough when hot. In my case, for some reason, the car would idle SUPER smooth (Lexus smooth) about halfway through the warm up cycle with the bad sensor.
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'89 silver 244DL in Milwaukee WI

How did you eventually figure out that the sensor was bad? Did you test the voltage? My tailpipe has sooted up considerably over the last two weeks and even though the rpm gauge sits reasonably at 750, the car is idling very rough.

Your super smooth idle, did that just happen for a minute or so, then go away?

Did FCP send you a new sensor?
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89 245 'loaded' with a Great Pyrenees

Measure the 02 sensor output when the car is warmed up-- does it fluctuate between 0.15 and about 0.85V over a few seconds?

The smelly exhaust is a clue. I would also look for OBD codes. Sounds like it is running really rich.

Good luck!

Simple things first - are you sure you screwed it in tight enough? Are you sure you connected the wires? And since you think it possible that someone could have siphoned gas from your car, maybe you need to run through another half-tank and measure the mileage.
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'93 244: 'A' cam 4 deg. advanced, 25/22 sways, custom heim endlinks, poly bushings, and a lot more styling customization than I care to recount.

...yeah the half mileage hit is extreme. I don't think even running without a sensor would create that harsh a difference. It's like five gallons just disappeared.

I'll fill up another half tank and see. I hope that's the answer.

Thanks Scor
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89 245 'loaded' with a Great Pyrenees

240 wagon - 02 sensor tested and not working right/considering changing for mileage and general car upkeep (too old for emissions testing requirements in my state).

*LOTS of responses saying "leave well enough alone" - my 1993 is newer to me w/ horrible maintenance if any - afraid I too, may get negatives from the sensor replacement (15.5+ mpg/city, and sensor MIGHT help - remember reading that sensor failure may affect other things???

Suggestions?!!! - Thanks

Look at your Bentley manual. You will need to do a couple of things. First, make sure that you have no exhaust leaks. Then find (beg borrow or steal) a dwell meter. Connect the dwell meter to the AMM sensor, per the Bentley manual, and adjust the mixture. You should see the fuel economy come back to where it should be. Mine is set with a dwell meter at 27 degrees. My mileage went from 23 and change to a regular 27, and on a trip without exceeding 2500 RPM, and not using the A/C, I recorded one tank at 29.3. This is with an Automatic transmission 245.

Bill
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Those who beat their swords into plowshares will surely plow for those who did not.

It's true. The correct AMM for the '93 (I have two '93s) is not adjustable, unlike my '84 which is (I adjust that one for ~300 ohms and my emissions pass with flying colors).
Since I only have an '84 and a '93 (nothing in-between), I don't know when the change to a non-adjustable AMM took place, but I'd guess that it's with the change to LH-2.4, or else when the ECU took over more control of the engine (such as spark advance/retard) with the substitution of a distributor sensor for a flywheel sensor.

Isn't that adjustment on older 240 AMM's? Car is a '89 and AMM is an 016 and there is nothing to adjust. No screw or anything...

No exhaust leaks before the O2 sensor (located before cat below car) at all...
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89 245 'loaded' with a Great Pyrenees

Yes,

Mine is a 1985 With the LH 2.2 system. I don't think that yours is adjustable. Is the Lambda Sensor the right one?

Bill
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Those who beat their swords into plowshares will surely plow for those who did not.

Lambda sensor?

The old O2 sensor is a Bosch 13034 and so is the new one.

The AMM is a Bosch xxxxxxxxxx016

Jason
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89 245 'loaded' with a Great Pyrenees

Just another name for the oxygen sensor.
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Those who beat their swords into plowshares will surely plow for those who did not.

Put the old one back in! :)
Charles

Oh man I know, I know...I'm an idiot! I threw the old one away after the new one seemed to smooth out the idle.
"Oh gee," I thought. "This new part works great!"
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89 245 'loaded' with a Great Pyrenees